A powerful political clan allied to the Philippine president, Gloria Macapagal-Arroyo, is suspected of masterminding an election massacre that left 57 people dead, police said today.

Four local commanders, including one provincial police chief, have already been relieved of their duties and confined to camp while being investigated for what the UN secretary general, Ban Ki-moon, called a "heinous crime".

As the full extent of the carnage emerged, domestic and international pressure is growing on Arroyo to find and punish those responsible.

In her first comments on the killings the president, who has declared a state of emergency in Maguindanao and another southern province, promised justice for the victims and declared a national day of mourning.

"This is a supreme act of inhumanity that is a blight on our nation," she said in a statement. "The perpetrators will not escape justice. The law will hunt them until they are caught."

Suspicion has fallen on the Ampatuans, a powerful political dynasty in the southern province of Maguindanao. Police are investigating reports that Andal Ampatuan Jr, a town mayor, was present when dozens of police and pro-government militia stopped the election convoy that was attacked.

Human Rights Watch in New York urged the government to start a fully independent investigation led by the Philippines' National Bureau of Investigation, given allegations of involvement by members of the security forces and local militias.

"Far too many people have been gunned down in the Philippines while President Arroyo has sat on her hands," said Elaine Pearson, deputy Asia director at Human Rights Watch. "The possible involvement of state forces in the Maguindanao massacre means that security personnel shouldn't be allowed to interfere in an independent investigation."

Local human rights officials voiced fears that the Philippine government would pull its punches in any investigation because the Ampatuans have helped deliver votes for Arroyo in past elections.

"I have run out of words to describe this. It's ignominious, bestial," Leila de Lima, who chairs the Philippines independent commission on human rights, told the Associated Press. She said there was "strong circumstantial evidence" implicating the Ampatuans.

"I am sure the president must be agonising now," she said, "but it's a great test of political will, how far the national authorities can go in terms of swift and decisive action. Anything less than that, well, people would be able to put them to task."

In an indication of how hard it will be for investigators, the justice secretary, Agnes Devanadera, admitted that the massacre had produced a chilling effect on provincial government prosecutors who fear that they might suffer the same fate as the victims.

"The prosecutors in Maguindanao cannot move because they are scared. Because of the many deaths, they do not want to return there. So we got prosecutors who are not from Maguindanao," Devanadera said, according to local media.Police today found 11 more bodies in a mass grave, bringing the death toll in Monday's attack on an election convoy to 57. Not all the bodies have been identified, but at least 18 of the victims are believed to be journalists, making Monday's attack the deadliest ever on the media anywhere in the world. Thirty-three of the victims were men and 24 were women, police said.

The dead included the family of Ismael Mangudadatu, who planned to challenge a member of the Ampatuan clan for the governor's office. After receiving death threats, Mangudadatu sent his wife and relatives to file his candidacy papers for elections taking place next year.

Mangudadatu believed that the presence of women and numerous journalists would ensure the safety of the convoy. But it was stopped by about 100 armed men, who herded them on to a remote hillside and attacked them with M-16 rifles and machetes. Some of the women were reportedly raped before they were killed.

Two vans and many bodies were thrown into a freshly dug pit and covered with earth by an excavator. A Reuters photographer at the scene said the vehicles were buried with dead men at the wheel.

Mangudadatu said four witnesses in his protection, whom he refused to identify, told him the convoy was stopped by dozens of gunmen loyal to the Ampatuan clan.

The army has disbanded a 200-member paramilitary force under the control of local officials and sent an extra 500 soldiers from a central island in the Philippines to reduce tension.

Suspicion has fallen on the Ampatuans because of the rivalry between them and the Mangudadatu family. Local media reports say the governor of Maguindanao, Andal Ampatuan, wanted his son, Andal Ampatuan Jr, the mayor of Datu Unsay, to succeed him next year.